Corticotropin-releasing factor (CRF) and the serotonergic (5-HT)-dorsal raphe nucleus (DRN) system have been independently implicated in the acute response to stress and in stress-related psychiatric disorders (e.g., anxiety, depression). During the last few years, we provided evidence that these two systems are anatomically and functionally linked in the response to stress. Thus, we demonstrated that CRF regulates DRN neuronal activity and 5-HT release in forebrain targets in response to acute stress. Moreover, we demonstrated that repeated stress results in cross-adaptation of the DRN-5-HT system to CRF and subsequent stress. The guiding hypothesis of this competing renewal is that CRF neuromodulation of the DRN-5-HT system underlies certain components of the behavioral limb of the acute stress response and that stress-induced plasticity in this function of CRF underlies psychiatric symptoms associated with repeated or chronic stress. The following AIMs integrate diverse approaches to test this hypothesis. AIM 1: will use in vivo microdialysis and electrophysiology to elucidate specific neuronal and neurochemical consequences of selectively activating CRF-R1 vs. CRF-R2 receptors within the DRN. AIM 2: will identify the anatomical substrates and circuitry by which CRF impacts on the DRN-5-HT system using immunohistochemistry, in situ hybridization and electron microscopy. AIM 3: will identify behavioral consequences of selectively activating CRF-R1 vs. CRF-R2 receptors within the DRN. AIM 4: will examine stress-induced adaptation of the DRN-5-HT system with regard to a) the role of CRF, b) the underlying mechanisms and c) whether neurochemical adaptation is causal to behavioral adaptation (using in vivo microdialysis, behavior and receptor autoradiography). Our previous work introduced the innovative idea that CRF-5-HT interactions are a pathophysiological focus of psychiatric disorders and a target of novel psychotherapeutic agents. The proposed studies will thoroughly characterize these interactions from a cellular to behavioral level. As such, they will enhance our understanding of the role of stress in psychopathology and indicate novel targets for the treatment of stress-related psychiatric disorders.